| interaction {base} | R Documentation |
interaction computes a factor which represents the interaction
of the given factors. The result of interaction is always unordered.
interaction(..., drop = FALSE, sep = ".")
... |
the factors for which interaction is to be computed, or a single list giving those factors. |
drop |
if drop is TRUE, unused factor levels
are dropped from the result. The default is to retain all
factor levels. |
sep |
string to construct the new level labels by joining the constituent ones. |
A factor which represents the interaction of the given factors.
The levels are labelled as the levels of the individual factors joined
by sep, by . by default.
The levels are ordered so the level of the first factor varies
fastest, then the second and so on. This is the reverse of
lexicographic ordering, and differs from :. (It is done
this way for compatibility with S.)
Chambers, J. M. and Hastie, T. J. (1992) Statistical Models in S. Wadsworth & Brooks/Cole.
factor;
: where f:g is similar to
interaction(f, g, sep=":") when f and g are factors.
a <- gl(2, 4, 8)
b <- gl(2, 2, 8, label = c("ctrl", "treat"))
s <- gl(2, 1, 8, label = c("M", "F"))
interaction(a, b)
interaction(a, b, s, sep = ":")